The notion of Kan extensions subsumes all the other fundamental concepts of category theory.

Kan extensions generalize the notion of extending a function defined on a subset to a function defined on the whole set. The definition, not surprisingly, is at a high level of abstraction. When specialised to posets, it becomes a relatively familiar type of question on 'constrained optimization'.

and comes in two varieties: the "left" Kan extension and the "right" Kan extension of along .

It amounts to finding the dashed arrow and the 2-cell in the following diagram:

Formally, the right Kan extension of along consists of a functor and a natural transformation which is couniversal with respect to the specification, in the sense that for any functor and natural transformation , a unique natural transformation is defined and fits into a commutative diagram

(where is the natural transformation with for any object of ).

The functor R is often written .

As with the other universal constructs in category theory, the "left" version of the Kan extension is dual to the "right" one and is obtained by replacing all categories by their opposites. The effect of this on the description above is merely to reverse the direction of the natural transformations (recall that a natural transformation between the functors consists of the data of an arrow for every object of , satisfying a "naturality" property. When we pass to the opposite categories, the source and target of are swapped, causing to act in the opposite direction).

This gives rise to the alternate description: the left Kan extension of along consists of a functor and a natural transformation which are universal with respect to this specification, in the sense that for any other functor and natural transformation , a unique natural transformation exists and fits into a commutative diagram:

(where is the natural transformation with for any object of ).

The functor L is often written .

The use of the word "the" (as in "the left Kan extension") is justified by the fact that, as with all universal constructions, if the object defined exists, then it is unique up to unique isomorphism. In this case, that means that (for left Kan extensions) if are two left Kan extensions of along , and are the corresponding transformations, then there exists a unique isomorphism of functors such that the second diagram above commutes. Likewise for right Kan extensions.

are two functors such that for all objects m and m' of M and all objects c of C, the copowers exist in A. Then the functor T has a left Kan extension L along K, which is such that, for every object c of C,

A functor possesses a left adjoint if and only if the right Kan extension of along exists and is preserved by . In this case, a left adjoint is given by and this Kan extension is even preserved by any functor whatsoever, i.e. is an absolute Kan extension.

Dually, a right adjoint exists if and only if the left Kan extension of the identity along exists and is preserved by .